Diaphragm electrolytic cells have been employed for a long time in alkali chloride brine electrolysis, such as sodium chloride electrolysis for the production of chlorine and caustic soda. This technology typically provides the deposition of a fibrous semipermeable diaphragm on a multiplicity of foraminous cathodic structures such as meshes or punched sheets and the installation thereof inside an electrolysis cell, intercalated to corresponding anodic structures consisting of valve metals members coated with a catalytic composition for chlorine evolution. Diaphragm fibres were originally based on asbestos, an optimal material from the point of view of permeability and chemical resistance in the usual process conditions, but whose utilisation entails well-known remarkable health hazards. In the course of the last years, attempts have been made to replace asbestos with synthetic fibres having substantially equivalent characteristics. As a matter of fact, the selection of materials of possible use is limited by the highly aggressive reaction environment. In order to obtain a sufficient resistance, in particular to nascent chlorine, at least partially fluorinated polymers were developed, which on the other hand present a rather high hydrophobic character. The latter is a detrimental feature which poses some safety concerns, since it prevents the diaphragm from being adequately soaked with the process electrolyte and from effectively separating the gaseous products formed in the two cell compartments (hydrogen at the cathodic compartment and chlorine at the anodic compartment). Polymer fibres must, therefore, be modified with suitable additives in order to impart the required wettability to the diaphragm.
One example of a commonly employed additive is zirconium oxide, which has been used in the past for asbestos diaphragms. There has been disclosed a zirconium and magnesium oxide-based coating to hydrophilise a polytetrafluoroethylene-based diaphragm, which is still a poorly satisfying solution, leading to a diaphragm imbibition of about 25% of what would be obtained with asbestos fibres.
A much better result can be achieved by directly producing a composite fibre, embedding or encrusting zirconium oxide particles inside, preferably fluorinated organic polymer fibres, as disclosed in U.S. Pat. No. 4,853,101, the contents of which are herein incorporated by reference in their entirety. The diaphragm of U.S. Pat. No. 4,853,101 is manufactured starting from an aqueous suspension of composite anisotropic fibres consisting of a preferably fluorinated polymer (for example PVDF or PTFE) having a consistent amount of zirconium oxide-based ceramic particles mechanically impacted on their surface. The suspension is subsequently deposited on a cathodic structure comprising a foraminous metallic body (for instance a mesh) acting as a filter for the suspension. To accelerate the process, the deposition is normally effected by applying a negative pressure on the opposite side of the cathode body by means of a vacuum pump. In order to stabilise the composite fibres on the cathode surface, an appropriate sintering treatment is carried out at such a temperature that the flowing of polymer particles is started, without reaching the melting or the decomposition point. The fluorinated polymers, for instance, are typically treated at temperatures between 320° C. and 380° C. for a time not exceeding three hours. In full operating regime conditions, the diaphragm of U.S. Pat. No. 4,853,101 presents functioning characteristics slightly inferior to asbestos diaphragms. Nevertheless, the attainment of optimum performances is very slow, delicate and cumbersome. Especially during the lengthy start-up phase, but to some extent also during the regular functioning, the inadequate gas separation due to the residual presence of preferential hydrophobic paths within the diaphragm structure or of capillary pores which the process electrolyte is unable to fill, causes some hydrogen leakage to the anodic compartment, with possible formation of explosive mixtures in the more extreme situations. In some cases, high surfactant concentrations are used in the start-up phase in order to accelerate diaphragm hydrophilization. Typically, the electrolyte is added with an amount up to 5% by volume of Zonyl® (a fluorinated surfactant produced by DuPont, USA), which favours the electrolyte penetration within the pores of the diaphragm, decreasing the hydrophobicity of the polymer fibre, but which is leached out in a short time without offering a definitive solution to the problem.
It would, therefore, be desirable to provide a synthetic separator for diaphragm chlor-alkali cells overcoming the above problems, with particular reference to improving the separation of gaseous products.